1. Field of the Invention
The present invention relates to a method and apparatus for emulating source instructions on a target information processing system and, more particularly, to a scheme for translating source instructions to target instructions executable on the target information processing system.
2. Description of Related Art
Information processing system or computing systems comes in many configurations. They include without limitation the generally known computers (such as personal computers), personal digital assistant, video game consoles, application specific control systems, and other systems and devices that incorporate a processing unit. For example, computers have general-purpose central processing units (CPU) which are designed to execute a specific set of instructions.
A CPU of one family, such as the Motorola™ 680X0™ family of processors, manufactured by Motorola, Inc., Phoenix, Ariz., executes instructions unique to this family, a CPU of another family, such as the Intel™ 80X86™ manufactured by Intel Corp., Sunnyvale, Calif., executes instructions unique to that family, and a PowerPC™ processor family, used in a Macintosh™ computer manufactured by Apple Computer, Inc., Cupertino, Calif., executes instructions specific to that family. These instructions comprise part of the operating systems of the respective computers in which the respective CPUs reside. Typically, application software programs are then written to run on the respective operating systems.
As another example, video game manufacturers, such as Sony™, developed a game player based on a proprietary hardware (processing) platform that runs only game software in a CD format which is specifically developed for its platform. Sony game software would not run on other hardware platforms. Game software based on other platforms would not run on Sony™ players.
A computer manufacturer will design a computer system with a given CPU and will want to maximize market penetration by having more rather than fewer applications software being able to run on its system. For example, as indicated above, Apple Computer manufactures the PowerPC-based Macintosh line of computers. Applications software that has been written to run on operating systems for the Motorola 680X0 family, for example, may not all run on the PowerPC-based machines. Accordingly, a field of technology has developed in which a given computer having one type of CPU, called a target, will include an emulator that allows the target computer to emulate the instructions, called the source, of another type of CPU. Thus, the target computer will have stored in memory source instructions that may be called in response to applications software, target instructions emulating the source instructions and executable by the target CPU, and an emulator that causes one or more target instructions to be executed in response to a given source instruction. Thus, the given computer can execute target instructions of its own machine, and through emulation execute source instructions.
Two main types of emulation strategies currently are available in the emulation field. The first strategy is known as “interpretation”, in which each source instruction is decoded in turn as it is addressed, causing a small sequence of target instructions then to be executed that emulate the source instruction. The main component of an emulator is typically an interpreter that converts each instruction of any program in machine language A into a set of instructions in machine language B, where machine language B is the code language of the target computer on which the emulator is being used. In some instances, interpreters have been implemented in computer hardware or firmware, thereby enabling relatively fast execution of the emulated programs.
The overhead, in terms of speed, for decoding each source instruction each time the source instruction is called is relatively small, but present for each instruction. Consequently, a given source instruction that is addressed and, hence, decoded, many times in the course of running a program will slow the execution time of the overall program being run, i.e., overhead will increase.
The other main emulation strategy is known as “translation”, in which the source instructions are analyzed and decoded. This is also referred to as “recompilation” or “cross-compilation”. It is well known that the execution speed of computer programs is dramatically reduced by interpreters. It is not uncommon for a computer program to run ten to twenty times slower when it is executed via emulation than when the equivalent program is recompiled into target machine code and the target code version is executed. Due to the well known slowness of software emulation, a number of products have successfully improved on the speed of executing source applications by dynamically cross-compiling portions of such program at run time into target machine code, and then executing the recompiled program portions. While the cross-compilation process typically takes 50 to 100 machine or clock cycles per instruction of the source, the greater speed of the resulting target machine code is, on average, enough to improve the overall speed of execution of most source applications.
The primary reason that overall execution speed is improved by cross-compilation is that most programs contain execution loops of instructions that are repeatedly executed hundreds, thousands, or even millions of times during a typical execution of the program. The source instructions are analyzed and decoded only once, i.e., the first time they are addressed, and the target instruction stream is generated and stored in memory, usually a RAM memory that may be a cache memory. By avoiding repeated interpretation of the instructions in such loops, substantial execution time is saved. Consequently, subsequent emulation of the same source instruction may be performed quickly because the decoding overhead is nonexistent.
While run time cross-compilation of source applications is well known to those skilled in the art, there are several areas in which existing cross-compilation systems have fallen short of their potential. For example, it is necessary to have a relatively large buffer or cache memory in the target computer. A block of memory in the target computer's memory address space is set aside to store the target instruction stream generated during the initial translation process. If this block is large enough to contain the entire translated target instructions, emulation will proceed at the maximum rate since, in response to a given source instruction, the RAM memory can be quickly addressed to access the corresponding sequence of target instructions for execution. Otherwise, there is translation overhead necessary to handle translation of the program in small sections during the execution of the source program. However, it often is expensive or prohibitive to set aside a large enough RAM memory. Furthermore, it may also be impossible to determine how large a RAM memory must be allocated in the target computer's memory address space to contain the translated program. Competition for RAM or cache memory space with other application software running on the same target machine may further limit the execution speed of source applications. For applications involving video graphics, it is important that continuity in the graphics is maintained by the emulation of the source application.
Consequently, a competing interest exists between the size of the RAM memory and the translation overhead. If the RAM memory is relatively large, the translation overhead will be relatively low, but at the high cost of memory. If the RAM is small, reducing memory cost, the translation overhead may be high due, for example, to continually writing newly translated code to the RAM. These drawbacks are more significant with low-end machines running on a slower processor and running on limited amounts of memory.
Accordingly, it is desirable to provide an improved system and method for dynamic recompilation that produces a more efficient set of translated code and uses less memory.